A Customer Case: When an Oil Leak Is Not Necessarily a Premature Cylinder Failure
Oil leakage from a hydraulic cylinder is often interpreted as a straightforward seal failure. When oil becomes visible around the piston rod, the immediate reaction may be to replace the rod seal and return the cylinder to service.
However, after hundreds of thousands of operating cycles, the correct engineering question is different:
Is the seal simply worn, or is another operating condition contributing to the leakage?
A technical case analyzed by the Vega Team involved a hydraulic cylinder that had been operating for approximately four years. The Customer reported an oil leak that appeared to come from the rod area. The cylinder was installed vertically and had completed approximately 700,000 cycles at an operating temperature of approximately 60°C.
The hydraulic pressure was relatively low: approximately 50 bar in one direction and 30 bar in the other.
After reviewing the operating conditions, the Vega Team considered that the leakage could be consistent with the high number of completed cycles. However, instead of simply recommending replacement of the rod seal, Vega suggested checking the piston rod surface carefully and evaluating a special rod-seal solution according to the actual operating temperature.
This case provides a useful lesson in hydraulic-cylinder maintenance: the point where oil becomes visible is not necessarily the root cause of the problem.
1. The Customer’s Problem
The Customer reported an oil leak from a hydraulic cylinder that had been in service for several years.
The cylinder was installed in a vertical position, and the Customer could not determine the exact source of the leakage. The initial impression was that oil was escaping along the piston rod.
The application conditions provided by the Customer included:
- approximately 700,000 operating cycles;
- approximately 60°C operating temperature;
- normal hydraulic oil;
- approximately 50 bar pressure in one direction;
- approximately 30 bar in the opposite direction.
These details are essential because seal performance cannot be evaluated independently of the actual operating conditions.
2. Why 700,000 Cycles Matter
A hydraulic cylinder that has completed 700,000 cycles cannot be evaluated in the same way as a new cylinder that begins leaking during its first hours of operation.
Every operating cycle causes sliding contact between the rod and the sealing system.
Over time, this repeated movement contributes to normal wear.
The Vega Team therefore considered that the reported leakage could be physiological in relation to the number of cycles performed.
This does not mean that leakage should simply be accepted.
Instead, it changes the diagnostic question from:
“Why has a new component failed?”
to:
“Has the sealing system reached a normal maintenance interval, or is another condition accelerating wear?”
3. The Rod Seal Is a Wear Component
The rod seal performs a demanding task.
During operation, the piston rod continuously moves through the sealing system. The seal must maintain contact with the rod while allowing smooth movement and preventing hydraulic oil from escaping.
Its operating life is influenced by several factors, including:
- number of cycles;
- rod speed;
- temperature;
- hydraulic pressure;
- hydraulic fluid;
- contamination;
- rod alignment;
- rod surface condition.
Consequently, a seal should not be considered independently from the rest of the cylinder and its application.
4. Why the Vega Team Recommended Checking the Rod Surface
One of the most important recommendations in the Customer case was to check the surface of the piston rod.
This is critical because the rod seal operates directly against this surface.
If the rod surface has deteriorated, simply installing a new seal may provide only a temporary solution.
During a maintenance inspection, engineers should therefore look for conditions such as:
- scratches;
- scoring;
- corrosion;
- surface damage;
- contamination;
- abnormal wear.
These are general engineering inspection points. The Customer documentation does not state that any of these defects were actually found on the rod in this specific case.
5. Do Not Confuse the Symptom With the Cause
An external oil leak is a symptom.
A damaged or worn seal may be the cause, but the seal may also be reacting to another condition.
For example:
Oil appears around the rod
↓
Rod seal no longer maintains effective sealing
↓
But why?
Possible explanations can include normal wear, unsuitable operating conditions, surface damage, contamination or mechanical problems.
The objective of troubleshooting is therefore not simply to identify which component is leaking, but to understand why it is leaking.
This principle is also discussed in Vega’s technical knowledge base, where recurring leakage is analyzed as a root-cause problem rather than simply a seal-replacement problem.
6. Temperature Is an Essential Parameter
The Customer reported an operating temperature of approximately 60°C.
Temperature directly influences the long-term behavior of sealing materials.
Seal selection should therefore consider not only dimensions but also:
- continuous operating temperature;
- hydraulic fluid;
- pressure;
- sliding speed;
- expected cycle life.
Vega’s official materials documentation confirms that hydraulic-cylinder sealing systems use different materials according to their function and that special sealing options are available for applications involving unusually high temperatures or specific fluids.
7. Vega’s Proposal: Special Rod Seals
The Vega Team did not simply recommend installing another standard replacement seal.
Based on the operating temperature, Vega indicated that it could propose special rod seals intended to increase the number of cycles between maintenance interventions.
This distinction is important.
The objective was not merely:
stop today’s leak
but rather:
increase the service interval before the next maintenance operation.
This is a more useful approach for high-cycle industrial applications.
8. Temperature Capability of the Proposed Solution
In the technical correspondence, the Vega Team indicated that the special sealing solution under consideration could operate at approximately 90–100°C, depending on the specific configuration.
The Customer reported an actual operating temperature of approximately 60°C.
Therefore, according to the information exchanged in this specific case, the proposed special sealing solution provided additional temperature capability compared with the reported operating condition.
The final seal selection should nevertheless always be based on the complete application parameters.
9. Operating Pressure Was Also Relevant
The Customer reported relatively low hydraulic pressures:
50 bar in one direction
and
30 bar in the other direction.
The Vega Team specifically considered the low operating pressure when evaluating the possibility of providing special seals.
This illustrates an important principle:
seal selection should never be based on temperature alone.
Pressure, temperature, fluid and movement must be considered together.
10. Seal Material Is an Engineering Decision
Hydraulic seals may appear to be relatively simple components, but their material properties strongly influence cylinder reliability.
Vega’s official materials page explains that its standard sealing system uses different materials for different functions, including PTFE + bronze seals, FKM O-rings and self-lubricating guide components, while special options are available for particular operating conditions.
The Vega technical knowledge base also emphasizes that temperature, hydraulic fluid, pressure, sliding speed and environmental conditions can all influence seal performance.
Therefore, choosing a seal should be considered part of the engineering process, not simply a spare-parts decision.
11. Why Replacing the Seal May Not Be Enough
Consider two maintenance approaches.
Approach A: Replace the Seal
Oil leakage is detected.
The maintenance team replaces the rod seal.
The cylinder returns to operation.
Approach B: Investigate the Cause
Oil leakage is detected.
The maintenance team records the operating conditions.
The rod surface is inspected.
The seal is examined.
Temperature, pressure, fluid and cycle count are verified.
A sealing system is then selected according to the actual application.
The second approach requires more investigation, but it can reduce the risk of recurring leakage.
12. The Importance of Rod Surface Quality
Vega’s official product documentation states that the rod is manufactured from chromed and hardened alloy steel, reflecting the importance of this component in hydraulic-cylinder performance.
In the V450CM family, Vega also describes the rod and piston as being manufactured from high-quality chromium-molybdenum steel.
The surface against which the rod seal operates is therefore a fundamental part of the sealing system.
A seal and its sliding surface should always be considered together.
13. High-Cycle Applications Require a Different Maintenance Strategy
For applications operating hundreds of thousands or millions of cycles, maintenance planning becomes increasingly important.
Instead of waiting for significant leakage, operators can monitor:
- cycle count;
- minor oil traces;
- rod condition;
- temperature;
- operating pressure;
- changes in cylinder behavior.
The objective is to move from purely corrective maintenance toward planned maintenance based on actual operating conditions.
14. Cycle Count Should Be Considered During Cylinder Selection
Engineers commonly specify:
- bore;
- stroke;
- force;
- pressure;
- mounting configuration.
For high-production machinery, another question should be included:
How many cycles is the cylinder expected to complete?
A cylinder expected to operate occasionally has very different service requirements from one expected to complete hundreds of thousands of cycles.
Expected cycle life can influence:
- seal selection;
- inspection frequency;
- spare-parts strategy;
- preventive maintenance intervals.
15. Hydraulic Fluid Compatibility
The Customer reported using normal hydraulic oil.
Fluid compatibility is another important consideration when selecting seals.
Vega’s technical documentation notes that different operating fluids may require different sealing solutions, and its materials page specifically mentions special options for water-glycol applications.
For the V450CM series, the official Vega page lists compatibility with ISO VG 46 mineral oil and water-glycol mixtures.
Whenever a non-standard fluid is used, compatibility should therefore be verified before selecting the sealing system.
16. The Cylinder Was Installed Vertically
The Customer specifically reported that the cylinder was installed in a vertical position.
This information was part of the application data provided to Vega.
However, the available Customer documentation does not establish the vertical installation as the cause of the leakage.
It should therefore be treated as an application parameter rather than a confirmed failure mechanism.
This distinction is important when documenting real technical cases: reported conditions and confirmed causes should not be mixed.
17. A Practical Diagnostic Procedure for Rod-Seal Leakage
When oil appears around the piston rod, a systematic investigation can follow this sequence:
Step 1 — Confirm the leakage point
Clean the cylinder and determine where the oil actually appears.
Step 2 — Record the cycle count
Determine how long the sealing system has been operating.
Step 3 — Verify temperature
Measure the actual operating temperature rather than relying only on the mold or machine temperature.
Step 4 — Verify pressure
Record pressure during both extension and retraction.
Step 5 — Identify the hydraulic fluid
Confirm compatibility with the sealing materials.
Step 6 — Inspect the rod
Check the sliding surface carefully.
Step 7 — Inspect the seals
Determine whether the wear appears consistent with normal service or suggests another problem.
Step 8 — Evaluate alternative sealing materials
If the application is demanding, consider whether a special sealing configuration could extend service life.
18. Why Actual Seal Temperature Matters
In mold applications, the mold temperature is not necessarily the same as the temperature experienced by the cylinder seals.
Heat may travel through:
- the mold plate;
- the cylinder body;
- the piston rod;
- mechanical connections.
Vega’s technical knowledge base emphasizes this distinction when analyzing high-temperature applications: engineers should determine the temperature that actually reaches the sealing system rather than assuming it is identical to the mold temperature.
This can prevent both under-specification and unnecessary over-specification of sealing materials.
19. When Special Seals Can Be Worthwhile
Special seals may be useful when the objective is to:
- increase maintenance intervals;
- improve temperature resistance;
- improve compatibility with a particular fluid;
- optimize performance for specific pressure conditions;
- improve reliability in high-cycle applications.
In the Customer case, the objective identified by Vega was specifically to increase the number of cycles between maintenance interventions.
That is a different objective from simply stopping an existing leak.
20. The Customer Case in Numbers
| Parameter | Reported condition |
|---|---|
| Operating period | Approximately 4 years |
| Operating cycles | Approximately 700,000 |
| Temperature | 60°C |
| Pressure, one direction | 50 bar |
| Pressure, opposite direction | 30 bar |
| Installation | Vertical |
| Hydraulic fluid | Normal hydraulic oil |
| Reported problem | Oil leakage |
| Suspected leakage area | Piston rod |
| Vega recommended inspection | Rod surface |
| Possible improvement | Special rod seals |
These values are taken from the Customer correspondence and Vega Team’s technical response.
21. From Repair to Reliability Improvement
A useful way of interpreting this case is to distinguish between repair and reliability improvement.
A repair asks:
Which component must be replaced?
A reliability investigation asks:
Why did this component reach the end of its useful service, and can the next maintenance interval be extended?
The Vega Team’s recommendation followed the second approach.
Rather than treating the seal as an isolated spare part, Vega considered the rod surface, operating temperature, pressure and cycle count before proposing a different sealing solution.
22. The Broader Engineering Lesson
The case demonstrates that hydraulic-cylinder reliability depends on the interaction of several elements:
Cylinder design + rod surface + seals + hydraulic fluid + pressure + temperature + cycle count + maintenance
Changing only one component without investigating the others can leave the real cause unresolved.
This is particularly important in injection molds, where downtime caused by a relatively small oil leak can interrupt an entire production process.
Conclusion
A Customer reported oil leakage from a vertically installed hydraulic cylinder after approximately 700,000 operating cycles. The cylinder had been operating for around four years at approximately 60°C, with hydraulic pressures of approximately 50 bar and 30 bar depending on the direction of movement.
The Vega Team considered that the leakage could be consistent with the high number of completed cycles. Nevertheless, Vega recommended checking the piston rod surface before treating the problem as a simple seal replacement.
Vega also proposed evaluating special rod seals according to the actual operating temperature, with the objective of increasing the number of cycles between maintenance interventions.
The main lesson is therefore straightforward:
When a hydraulic cylinder begins leaking after a high number of cycles, replacing the rod seal may solve the symptom, but a complete diagnosis should also consider rod condition, operating temperature, pressure, hydraulic fluid and accumulated cycle life.
The best maintenance solution is not necessarily the one that simply stops today’s leak. It is the one that identifies the operating conditions behind the problem and helps extend the interval before the leak occurs again.
- Oil Leakage from the Rod Seal: Why Hydraulic Pressure Is Not Always the Real Cause — This iCVEGA technical article explains why oil leakage around the rod seal should not automatically be attributed to hydraulic pressure and why identifying the actual root cause is essential before replacing the seal.
Oil Leakage from the Rod Seal: Why Hydraulic Pressure Is Not Always the Real Cause - Materials and Components for Hydraulic Cylinders — This official Vega Cylinders page provides information about hydraulic-cylinder materials and components, including sealing systems, guide components and special solutions for demanding operating conditions.
Materials and Components for Hydraulic Cylinders - V450CM Heavy-Duty Short-Stroke Compact Hydraulic Cylinders — This official Vega Cylinders page presents the V450CM range, including its construction, technical characteristics, operating conditions and typical applications in injection molds.
V450CM Heavy-Duty Short-Stroke Compact Hydraulic Cylinders
